1. Field of the Invention
The present invention relates to a light-emitting device, and more particularly, to a light-emitting unit with enhanced thermal dissipation and method for fabricating the same. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for dissipating heat from a light-emitting unit that uses a light emitting diode as a light source.
2. Discussion of the Related Art
In general, a light-emitting device can be used as a high-brightness light source for flashlights, a backlight source for portable electronic devices, a light source for electronic display boards, an illumination light source for switches, a beaconlight, a traffic signal light, and so on. For example, a light-emitting device can be a plurality of light-emitting units in a light-emitting package. Such a package can be used as a backlight in portable phones, camcorders, digital cameras, and PDAs.
FIG. 1 is a perspective view of a related art light-emitting package. FIG. 2 is a cross-sectional view taken along line A–A′ in FIG. 1. Referring to FIGS. 1 and 2, a light-emitting package 60 includes a printed circuit board (PCB) 3, and a plurality of light-emitting units 1 installed on the PCB 3. Each of the light-emitting units 1 includes a housing 7, a light emitting diode (LED) 5, a heat sink member 9 attached inside the housing 7 and attached to the LED 5 for dissipating heat generated by the LED 5, a molding member 11 formed on the housing 7 to cover the LED 5, and first and second leads 13 and 15 for supplying electric power to the LED 5.
More specifically, the first leads 13 are electrically connected to a first metal electrode 25a and the second leads 15 are commonly connected to a second metal electrode 25b of the LED 5. These connections can be implemented by wire bonding. Although not shown in FIGS. 1 and 2, the LED 5 is attached to the heat sink member 9 with an electrically insulating adhesive.
As shown in FIG. 2, the PCB 3 includes a heat sink plate 21, an insulating layer 23 formed on the heat sink plate 21, and a patterned metal electrode layer formed on the insulating layer 23. The metal electrode layer includes first and second metal electrodes 25a and 25b formed on both side surfaces of the PCB 3, and a third metal electrode 25C formed on a center surface of the PCB 3. The PCB 3 has a rectangular shape extending in a lengthwise direction. The first to third electrodes 25a to 25c also extend in the lengthwise direction. Insulating members 27 for preventing an electrical short are positioned between the first metal electrode 25a and the third metal electrode 25c as well as between the second metal electrode 25b and the third metal electrode 25c. 
The light emitting units 1 is are attached to the third electrode 25c with a thermal conductive adhesive 17. The first and second leads 13 and 15 of the light-emitting units 1 are commonly attached respectively to the first and second metal electrodes 25a and 25b with a silver paste 19. In the related art light-emitting package 60, electric power supplied to the first and second metal electrodes 25a and 25b is commonly applied to the light-emitting units 1 through the first and second leads 13 and 15. This applied electric power causes all of the light-emitting units 1 to emit light.
An LED does not have perfect light-emission efficiency in converting electrical energy to light energy. Accordingly, some of the supplied electrical power is converted into heat. This heat increases the operating temperature of the LED so as to degrade the operating characteristics of the LED.
The operating temperature of the LED is inversely proportional to the energy bandgap, and the energy bandgap is inversely proportional to the wavelength of light emitted from the LED. Accordingly, as the operating temperature of the LED increases, the energy bandgap becomes narrower, and thus the wavelength of the emitted light increases. Therefore, when an LED emitting blue light has an increase in its operating temperature, it may emit green light, rather than blue light. This phenomenon is called “a color shift”. Consequently, when the heat generated by the LED is not rapidly dissipated to the outside, a desired-color light cannot be obtained due to color shift by the LED. Further, the brightness efficiency of light emitted from an LED decreases as the operating temperature of the LED increases.
In the light-emitting package 60, heat generated from the LEDs 5 is transferred by the heat sink members 9 to the heat sink plate 21 such that the heat can then be dissipated to the outside. However, there is a limitation as to how much heat can be rapidly transferred from the light-emitting units 1 to the heat sink plate 21 through the heat sink members. Thus, there is a limited amount of power that can be provided to the LEDs before the operating characteristics of the LEDs degrade. Also, there is a limit to how much the overall thickness of the light-emitting package can be reduced because of the thickness of the heat sink member 9 in the light-emitting unit 1 together with the thickness of the PCB 3.